The present invention relates to a radiation-polymerizable mixture containing as essential constituents: (a) a compound which has at least two terminal ethylenically unsaturated groups and which can form a crosslinked polymer by means of free radical initiated chain addition polymerization (b) a polymeric binder and (c) a radiation activatable polymerization initiator. The radiation polymerizable mixture of the invention is particularly useful as a dry transfer photoresist material in the production of solder masks.
Single-faced, double-faced, and particularly through-hole printed circuit boards are provided with a solder mask before they are soldered. The application of such masks leaves only the soldering points exposed, whereby tin is saved on soldering. In addition, some of the soldering heat is kept away from the workpiece, during the soldering process, so that heat-sensitive components with which the printed circuit boards have been equipped are not damaged during the soldering. Last but not least, solder masks have the important function of covering all those areas of the circuit diagram which must not come into contact with the solder so as to avoid, for example, the possible formation of conducting bridges between individual conducting paths, which are fatal for a circuit system. To satisfy these quite complex requirements, numerous processes and products in various forms have been used in the past.
It is clear from the foregoing explanation that solder masks must have an image pattern which is oriented according to the prescribed conducting paths. A method widely used in practice is to apply solder masks by screen printing. This method also makes it possible to apply masks which have a relatively high layer thickness, thereby ensuring that the conducting paths are well shielded and embedded. Masks produced by the screen printing process come up against a limit which is inherent in the system, namely, when the distances between conducting paths are reduced, the relatively coarse image structures of the screen print are no longer adequate for microconductor technology.
In recent years solder masks have, therefore, been produced to an increasing extent by photoresist techniques. As in the production of printed circuits, thermoplastic photopolymerizable layers are laminated by means of pressure and heat to the printed circuit board and hardened by imagewise exposure at those places where the printed circuit board is to be covered. The solder mask is obtained by washing away the unexposed parts of the layer. This process makes possible higher image resolutions than the screen printing process.
Suitable materials of this type have been described, for example, in U.S. Pat. No. 4,278,752. They comprise photopolymerizable layers which contain a certain amount of bonded halogen to improve the flame-resistance. Published European Patent Application No. 15,004 describes a similar material which can be developed dry by mechanically separating exposed and unexposed layer areas ("peel-apart process"). Finally, U.S. Pat. No. 4,230,793 describes a light-hardenable material for the same purpose in which the light-sensitive compounds used are referred to as light-sensitive epoxy resins but are not described in more detail.
Photopolymerizable mixtures known and used for producing photoresist masks are transferred dry and with heating onto the printed circuit board to be masked, and they must, therefore, necessarily be thermoplastic. However, for use as solder masks this property is disadvantageous since the masks must withstand temperatures above 200.degree. C. without decomposition and without melting or even softening too much. Although the photopolymer layer is hardened substantially in the exposed places by crosslinking polymerization, and although this light-hardening can be further enhanced in a known way by re-exposure of the developed image stencil, all known photopolymerizable layers still have a basic softening tendency on heating.
To overcome this disadvantage, it has been proposed in U.S. patent application Ser. No. 366,379 (filed Apr. 7, 1982) to add a bis-epoxy compound to the photopolymerizable mixture for producing the solder mask and to post-cure the light-hardened image stencil by heating after developing. For this purpose, the epoxy compound is dissolved in a customary manner in the coating mixture. This process produces very heat-stable solder masks. However, the photopolymerizable mixture has the disadvantage that its shelf life is shorter than that of an otherwise identical mixture which does not contain any epoxy compound.